1. Field of the Invention
This invention relates to an oxygen sensor, and more particularly, to a solid-state and Clark type oxygen sensor used to measure, by way of current response, the concentration of oxygen dissolved in a solution.
2. Description of the Prior Art
Oxygen electrodes used as oxygen sensors may be classified roughly into two types. These are:
(i) an internal liquid-retaining type having an internal electrolyte chamber separated from the liquid specimen such as by a polymeric membrane, and
(ii) a separating type in which a noble metal such as platinum is coated with a glass or polymeric membrane that is selectively gas-permeable.
However, the internal liquid retaining-type oxygen electrode (i) is disadvantageous in that (1) the membrane is readily damaged, (2) the internal electrolyte chamber is readily contaminated, and (3) miniaturization of the electrode is difficult. For these reasons, the separating-type electrode (ii), particularly solid-state electrodes, are drawing attention for use as oxygen sensors in the medical field.
One example of a solid-state electrode which can be mentioned is one obtained by depositing an oxygen-selective separating membrane on the surface of platinum. However, the electrode has the following drawbacks:
(1) The separating membrane is required to have a membrane thickness of 0.5-50 .mu.m in order to enhance its separating performance. Consequently, time is required for the oxygen to reach the surface of the platinum, thus resulting in a slow speed of response (usually 5 min).
(2) When immersed in a liquid specimen for an extended period of time in order to perform a continuous measurement, the electrode is influenced by interfering ions in the specimen. This means that a calibration curve prepared in advance can no longer be used.
(3) As a consequence of (2) above, long-term measurement requires that the electrode be extracted from the system and recalibrated. Thus, difficulties are encountered in terms of use.
In an effort to solve the foregoing problems, Japanese Pat. Application Laid-Open (KOKAI) No. 60-52759 proposes an oxygen sensor using electrically conductive carbon in place of platinum and a metal complex, which serves as a catalyst for the reduction reaction of oxygen, as an oxygen-selective separating membrane, with the metal complex being deposited on the surface of the electrically conductive carbon.
However, since the membrane of this oxygen sensor is formed by e.g. coating the conductive carbon with a solution of the metal complex followed by drying, the metal complex tends to elute into the liquid specimen. Though such elution is prevented by providing the sensor with an elution preventing layer such as Nafion R , the latter interacts with charged substances other than oxygen, as a result of which the oxygen sensor loses its selectivity.
An oxygen sensor available on the market is of the Clark type, in which silver/silver chloride is used as a reference electrode, platinum or platinum black is used as a working electrode, the electrodes are submerged in an alkaline solution such as a KOH solution, and the outside is coated with a silicone film. However, shortcomings encountered with this sensor include leakage and contamination of the internal liquid chamber of the electrode.
For continuous monitoring in a circulating system, the need for smaller, more durable electrodes having such characteristics as temperature compensation is emphasized. Recently, coated wire-type oxygen sensors in which a platinum electrode is directly coated with a cellulose membrane or other polymeric membranes have appeared on the market in the form of electrodes for testing purposes. These sensors exhibit durability problems and the period over which they can be used is shortened in areas of high PO.sub.2. (In an oxygenator, for example, this can be as high as 350 mmHg-760 mmHg, depending upon the particular case.) This can be thought of as being a problem in terms of durability. An oxygen sensor usable in a circulating system has not yet been marketed or developed.